BDE, that's why we have security by my shull in NMB.
I still say that trained attendee's that carry is the best safety.
We are aware of the dangers, sadly parents and or kids don't pay attention to security advice. I have said this before, and I'll repeat it, lo aleinu, the congregating outside shull after davening is a security concern.
Every shull needs effective security barriers to prevent car ramming attacks.
Every shull needs an active shooter lockdown, I said this one year ago when we moved to our new building, there should be a security team that should clear the outside of the building, alarms should go off, and the shull go on lockdown in 30-60 seconds
Less practical but highly effective would be to have a trained sniper on shull roof of in emergencies.